“20 years ago today a world that I had lived in alone was suddenly open to others,” the author, 51, wrote. “It’s been wonderful. Thank you. #HarryPotter20.”

J.K. Rowling attends the 70th EE British Academy Film Awards (BAFTA) at Royal Albert Hall in London on February 12, 2017.John Phillips/Getty Images

Harry Potter and the Philosopher’s Stone, which was later retitled Harry Potter and the Sorcerer’s Stone in the U.S., attracted readers of all ages when it was published in June 1997. The fantasy series quickly became a worldwide phenomenon, spawning a total of seven books and eight films. Rowling is also behind the spinoff movie Fantastic Beasts and Where to Find Them, and the acclaimed stage play Harry Potter and the Cursed Child.

Rowling’s longtime Twitter nemesis Piers Morgan was less than excited about the 20th anniversary of the Potter series, though. He replied to Rowling’s tweet, “20 years of conning the easily mesmerized masses with execrable literary hogwartwash. Congrats,” adding emojis of a nauseated face and a thumbs down.

Despite the British journalist’s lack of enthusiasm, plenty of fans celebrated the milestone day, prompting #HarryPotter20 to become the No. 1 trending topic on Twitter on Monday afternoon. “20 years! I can’t believe it! Thank you for changing the literary, cinematic and theatrical World with #HarryPotter,” one reader wrote. Another fan added, “I want to thank @jk_rowling I couldn’t imagine a world without HP.”